Materials within containers not readily susceptible to visual scrutiny, or, alternatively, materials that may be carried on the person of a human or on another animate subject, are routinely inspected by measurement of x-rays scattered by the materials from an irradiating x-ray beam. The characteristics of a material which might be the object of non-invasive inspection and which lend themselves to detection using the device and method taught by the invention include, but are not limited to, electron density, atomic number, mass density, linear dimensions and shape. These characteristics are unveiled by taking advantage of the various physical processes by which penetrating radiation interacts with matter. Additionally, scattered penetrating radiation may be used for imaging contents concealed within a vehicle or other container. Scattering in the backward direction may be referred to as backscatter, and is particularly implicated in the teachings of the present invention.
Penetrating radiation refers to electromagnetic radiation (or radiation of massive particles, such as neutrons) of sufficient energy per particle to penetrate materials of interest to a substantial and useful degree and include x-rays and more energetic forms of radiation. The interaction of such radiation with matter can generally be categorized as either scattering or absorption processes. Both types of process remove x-ray photons from a collimated (i.e., directional) beam; scattering processes do so by deflecting photons into new directions (usually with loss of energy), while absorption processes simply remove photons from the beam. As used herein, the term “x-ray” may be used as exemplary of penetrating radiation generally.
Description of the rudiments of various mobile inspection systems may be found in U.S. Pat. No. 5,764,683, issued Jun. 9, 1998, and in U.S. Pat. No. 7,099,434, issued Aug. 29, 2006, both of which incorporated herein by reference. As used in this description and in any appended claims, the term “source” is used in a broad sense to encompass the entirety of the apparatus used to generate a beam of penetrating radiation that is used to irradiate the object under inspection. The source is taken to include the generator of penetrating radiation (the “source”, in the narrow sense) which may include an x-ray tube or a radio-isotope. It is, furthermore, to be understood that the term “source” as used herein and in any appended claims, and as designated generally by numeral 30 in the drawings, refers to the entirety of the apparatus used to generate beam 24, and may have internal components that include, without limitation, apertures, choppers, collimators, etc.
Scatter imaging in which the x-rays scattered by a material (typically in a generally backward direction) are employed offers several unique inspection capabilities and operational features. Scatter imaging allows images to be obtained even when the imaged object is accessible from only one side. Moreover, since the scatter signal falls off quite rapidly with increasing depth into the object, backscatter images effectively represent a “slice” of the object characteristic of the side nearest to the x-ray source, thereby reducing problems of image clutter that may confound transmission images. The Compton effect, which dominates x-ray scatter in the energy range typically employed in accordance with the present invention, dominates the interaction of x-rays with dense low-atomic-number (low-Z) materials. Narcotic drugs tend to produce the bright signatures in a backscatter image, as do organic explosives, making backscatter imaging a useful imaging modality for bomb or drug detection. Finally, alignment requirements of the x-ray beam with detectors or collimation devices are less exacting than for transmission imaging thereby enabling rapid deployment in a wide range of inspection scenarios.
Flying-spot technology makes possible the acquisition of images using detectors specifically positioned to collect the scattered x-rays. In a typical flying-spot system, a thin “pencil beam” of x-rays is rapidly and repetitively swept through a source-centered track of beam paths that are arranged to intercept the object under inspection. At the same time, the inspection system moves relative to the inspected object at a substantially constant, slower speed along a path perpendicular to the track of the swept pencil beam. (It is to be understood that whether the source or the object moves relative to a local rest frame is immaterial to the present invention as claimed.) In this way, the pencil beam is made to traverse the object in point-by-point raster fashion, and the entire object is scanned as it passes through the fan plane over a period ranging from a few seconds to a few minutes depending upon the length of the object and the relative velocity of the object and the source.
FIG. 1 depicts a prior art mobile backscatter inspection system, such as described in U.S. Pat. No. 7,099,434, in which context embodiments of the present invention are advantageously applied. Backscatter detectors 100 are mounted on a mobile platform 10, or conveyance, typically capable of road travel, that traverses a large object to be inspected such as a vehicle or a cargo container 12. Conveyance 10 is characterized by an enclosure 14, here, the skin of a van, shown, in cutaway view, to enable depiction of other components of an inspection system. The conveyance can have many alternate embodiments, including but not limited to gasoline, diesel, electric, propane, battery, fuel-cell, or hydrogen-powered motor vehicles (including vans, trucks, or similar), tracked vehicles, sleds, trailers, cranes, or other equipment that can be put into motion, preferably self-propelled, but also including vehicles tethered and pulled such as under electric power.
Contained within enclosure 14 of conveyance 10 is a source 30 including x-ray tube 32 (shown in FIG. 2) and chopper 34. Rotating hoop 34, with aperture 38, emits a pencil beam 24 (also referred to, herein, as an “outgoing beam,” or “illuminating x-ray beam,” or “primary beam”), thereby enabling inspection of object 12.
Various means are known in the art for mechanically or electronically sweeping a beam of penetrating radiation, including, for example, the rotating chopper wheel 34 depicted in FIG. 2, or electronic scanning as described in detail, for example, in U.S. Pat. No. 6,421,420, issued Jul. 16, 2002, which is incorporated herein by reference. In embodiments employing a mechanical rotating chopper wheel 34, as the chopper wheel rotates in the direction of arrow 22, outgoing beam 24 of penetrating radiation emitted from the target of x-ray tube 32 passes successively through a plurality of channels.
Detectors 100 detect penetrating radiation from source 30 that has interacted with, and scattered from, contents of the inspected object 12, are carried by conveyance 10 and are typically enclosed within enclosing body 14 and concealed from view from outside the conveyance. They may also be carried outside the conveyance for particular applications within the scope of the present invention, as taught in U.S. Pat. No. 5,764,683. Detectors 100 are electrically coupled to processor 40, which receives and processes scatter signals, to render images of inspected object 12 and its contents, and/or to compute material characteristics of the contents of inspected object 12.